In the recovery of oil from oil-containing formations, it is usually possible to recover only minor portions of the original oil-in-place by so-called primary recovery methods which utilize only natural forces. To increase the recovery of oil a variety of supplementary recovery techniques are employed. These techniques include water flooding, miscible flooding, thermal recovery, and steam flooding.
A problem that arises in various flooding processes is that different strata or zones in the reservoir often possess different permeabilities. Thus, displacing fluids enter high permeability or "thief" zones in preference to zones of lower permeability. Significant quantities of oil may be left in zones of lower permeability. To circumvent this difficulty the technique of profile control is applied to plug the high permeability zones with polymeric gels and thus divert the displacing fluid into the low permeability, oil rich zones.
Polymeric gels are used to improve the sweep efficiency in reservoirs by reducing the loss of injection fluids into previously swept zones and diverting them to oil-rich zones. Zones previously swept by injection fluids are called thief zones. The aim of polymer gel treatments is to block the thief zones. A polymer and cross-linker mixture is injected into the thief zones and allowed to cross-link and form a gel.
Gel treatments typically involve the injection of a polymer and a cross-linker mixture into the wellbore at a constant injection rate. Injection pressure increases continuously during a gel treatment. The injection pressure increase can be attributed to moving a large bank of viscous liquid and to the increase in viscosity due to gelation. However, the injection pressure must be maintained below either the formation parting pressure or the formation fracture extension pressures.
Formation parting is undesirable since it opens up highly conductive fractures in the thief zones. Since the goal of the gel treatment is to plug the thief zones, it is undesirable to create fractures in the thief zones. These fractures would increase the size of the thief zone and cause a greater amount of gel to be utilized in order to close them off. For this reason, gel injection pressure is maintained below the formation parting pressure. In some instances, the wellbore may already have been fractured prior to gel injection. The fractures may have been created by a hydraulic fracturing process or due to the natural stresses in the reservoir. In this case, gel injection must be accomplished without exceeding the fracture extension pressure. The goal is to avoid further extension of the existing fractures during gel placement. The current practice is to maintain the gel injection pressure below either the fracture initiation or fracture extension pressure depending upon the existence of fractures near the wellbore. This pressure constraint often results in the injection of a smaller volume of gel than was predetermined which results in an area in the thief zone being untreated with gel.
Therefore, what is needed is a method of injecting a gel into a thief zone so as to place a predetermined or designed amount of the gel into a thief zone without fracturing a formation or without extending existing fractures in a formation.